My understanding of the Cat (I just had it replaced on my 1993 850 GLT--it was out for a year but I finally had to replace it to pass CA Smog for just over $700.00) is that it can be non-functioning without affecting performance--it was described to me in these terms: Eventually it will begin to disinegrate and will cause loss of power etc., like a sponge that eventually breaks apart and clogs the system.

I am sure someone else has more wisdom on this than I, but that's my 2 cents.

Good luck